Apparatus and method of active flutter control

Abstract

A system for controlling aeromechanical instability or flutter in turbofan engines having fan blades employs a sensor, such as an off-blade static pressure sensor or proximity detector mounted on a turbofan engine at an inlet of a rotor of the engine for generating a signal to detect resonance of the turbofan blades at frequencies associated with flutter. A controller is coupled to the sensor for generating by spatial Fourier decomposition from the sensor signal a command signal comprising a real time amplitude component and a spatial phase of disturbances of a predetermined nodal diameter and coincident with a natural frequency of resonance of a predetermined structural mode of the fan blades in the stationary frame. An actuator, such as a bleed valve or acoustic speaker, is mounted on the turbofan engine for damping flutter dynamics in response to the amplitude of the command signal.

What is claimed is:  
     
       1. A system for damping the aeromechanical instability of stall flutter in a turbofan engine having a plurality of blades having natural frequencies of resonance and structural modes associated with stall flutter spaced substantially equidistant from each other about a rotational axis, the system comprising: 
       a flutter sensor remotely positioned from turbofan blades at an inlet of a rotor of the engine for generating a sensor signal indicative of resonance of the turbofan blades at frequencies associated with stall flutter;  
       a controller coupled to the flutter sensor for generating from the sensor signal a command signal including a real time amplitude component and a spatial Fourier component (SFC) of disturbances of a predetermined nodal diameter and coincident with a natural frequency of resonance of a predetermined structural mode of the fan blades; and  
       an actuator in communication with the controller for modulating air pressure in response to the command signal of the controller in order to dampen stall flutter dynamics.  
     
     
       2. A system as defined in claim  1 , wherein the nodal diameters of the aeromechanical instability to be stabilized are≦4. 
     
     
       3. A system as defined in claim  1 , further including a bandpass filter for filtering the SFC of the command signal. 
     
     
       4. A system as defined in claim  3 , wherein the bandpass filter receives the 0th spatial Fourier component of the command signal, and passes a filtered spatial Fourier component of the command signal in the frequency range of about 250 Hertz to about 310 Hertz. 
     
     
       5. A system as defined in claim  3 , further including a signal amplitude scaler for scaling an amplitude of the filtered SFC of the command signal by a gain factor, and wherein the scaled signal is received by a control input of the actuator to open the actuator to a predetermined offset position. 
     
     
       6. A system as defined in claim  5 , wherein the gain factor is about −20. 
     
     
       7. A system as defined in claim  1 , wherein the flutter sensor is a proximity detector for detecting when each blade of the turbofan passes the proximity detector. 
     
     
       8. A system as defined in claim  7 , wherein the proximity detector is an active eddy current detector. 
     
     
       9. A system as defined in claim  1 , wherein the flutter sensor is a static pressure sensor for detecting changes in localized pressure near the blades caused by blade resonance at frequencies associated with stall flutter. 
     
     
       10. A system as defined in claim  9 , wherein the controller updates data received from the static pressure sensor at a rate of about 3000 Hertz. 
     
     
       11. A system as defined in claim  9 , wherein the static pressure sensor may include additional static pressure sensors distributed circumaxially about the inlet of the rotor, the number of static pressure sensors being≧(2* nodal diameter of the aeromechanical instability to be stabilized)+1. 
     
     
       12. A system as defined in claim  1 , wherein the actuator is a high-response bleed valve for altering internal pressure of the turbofan engine to dampen stall flutter dynamics in response to the command signal of the controller. 
     
     
       13. A system as defined in claim  1 , wherein the actuator includes an acoustic speaker for controlling stall flutter. 
     
     
       14. A system as defined in claim  1 , wherein the flutter sensor is a proximity detector for determining the actual arrival time of a turbofan blade, and further comprising a digital counting circuit for calculating a blade tip deflection estimate signal based on the difference between the expected and actual arrival times of a turbofan blade, the digital counting circuit including the proximity detector for determining the actual arrival time of a turbofan blade, and an expected arrival time sensor for determining the expected arrival time of a turbofan blade. 
     
     
       15. A system as defined in claim  14 , wherein the proximity detector is an active eddy current detector. 
     
     
       16. A system as defined in claim  14 , further comprising an observer circuit for estimating the aeromechanical modal content of the turbofan blades, the observer circuit including: 
       a nodal diameter construct sub-circuit for generating a nodal diameter estimate signal; and  
       a filter sub-circuit for filtering the command signal.  
     
     
       17. A system as defined in claim  14 , further including an inverse discrete Fourier transform (DFT) circuit coupled to the actuator for relaying command signals to the actuator. 
     
     
       18. A system as defined in claim  17 , wherein the actuator includes at least one volumetric source. 
     
     
       19. A system as defined in claim  18 , wherein the inverse DFT circuit has a plurality of outputs coupled to the at least one volumetric source. 
     
     
       20. A system as defined in claim  18 , wherein the at least one volumetric source includes an acoustic speaker. 
     
     
       21. A system as defined in claim  18 , wherein the at least one volumetric source is a plurality of volumetric sources arranged circumaxially about the turbofan. 
     
     
       22. A method of damping the aeromechanical instability of stall flutter in a turbofan engine having a plurality of blades having natural frequencies of resonance and structural modes associated with stall flutter spaced substantially equidistant from each other about a rotational axis, comprising the steps of: 
       sensing blade resonance associated with stall flutter at a location outwardly from the turbofan blades at an inlet of a rotor of the engine and generating a sensor signal indicative of the inception of stall flutter;  
       generating from the sensor signal a command signal including a real time amplitude component and a spatial Fourier component (SFC) of disturbances of a predetermined nodal diameter and coincident with a natural frequency of resonance of a predetermined structural mode of the fan blades; and  
       damping stall flutter dynamics in response to the command signal.  
     
     
       23. A method as defined in claim  22 , wherein the step of sensing includes sensing the static pressure at a location outwardly from the turbofan blades and generating a pressure signal to detect static pressure variations associated with the inception of stall flutter. 
     
     
       24. A method as defined in claim  22 , wherein the step of sensing includes a proximity detector which determines when the blades pass the proximity detector. 
     
     
       25. A method as defined in claim  22 , wherein the step of sensing includes a static pressure sensor provided outwardly from the blades for sensing localized pressure variations due to resonance of the blades at frequencies associated with stall flutter. 
     
     
       26. A method as defined in claim  22 , wherein the step of damping includes a high response bleed valve for altering localized pressure near the blades in response to the command signal. 
     
     
       27. A method as defined in claim  23 , wherein the step of damping includes an acoustic speaker for altering localized pressure near the blades in response to the command signal. 
     
     
       28. A method as defined in claim  22 , wherein the step of sensing is updated at a rate of about 3000 Hz. 
     
     
       29. A method as defined in claim  22 , further including the step of bandpass filtering the SFC of the command signal. 
     
     
       30. A method as defined in claim  29 , wherein the step of filtering includes receiving the 0th spatial Fourier component (SFC) of the command signal and passing the SFC of the command signal in the frequency range of about 250 Hz to about 310 Hz. 
     
     
       31. A method as defined in claim  22 , further including the step of scaling the filtered SFC of the command signal by a gain factor to form a scaled signal, and the scaled signal is received by a control input of the actuator to open the actuator to a predetermined offset position. 
     
     
       32. A method as defined in claim  31 , wherein the step of scaling includes scaling with a gain factor of about −20. 
     
     
       33. A method as defined in claim  22 , wherein the step of sensing includes sensing the static pressure at a location outwardly from the turbofan blades, generating a pressure signal to detect static pressure variations associated with the inception of flutter, sensing an expected arrival time, and generating a blade tip deflection signal indicative of the inception of flutter. 
     
     
       34. A method as defined in claim  33 , further including the step of filtering the command signal. 
     
     
       35. A method as defined in claim  34 , wherein the step of filtering includes estimating the aeromechanical modal content of a turbofan blade row based on the blade tip deflection signal, generating a nodal diameter estimate signal, and filtering out modes not associated with stall flutter. 
     
     
       36. A method as defined in claim  34 , wherein the step of damping includes altering the mass flow near the blades by at least one volumetric source in response to the command signal. 
     
     
       37. A method as defined in claim  36 , wherein the volumetric source is an acoustic speaker actuator.